Refinement of Low Carbon Bainite Structure by Deformation in Un-recrystallized Austenite Region

Abstract

Effects of deformation in unrecrystallized austenite region on isothermally transformed bainite structure were investigated in low carbon Nb-B bearing steels.
SEM observation shows, in a case of non-deformation, a number of lath-like bainitic ferrite plates form in parallel from austenite grain boundaries, showing a typical upper bainite structure. A 30%-deformation has little influence on the length of bainitic ferrite plates, although it shapes them like a sheaf of bows. In contrast, a 50%-deformation significantly decreases their length. However, it is confirmed that those bainitic ferrite plates are also surrounded by two sets of habit planes close to {451}_a, which suggests the bainitic ferrite transformed from heavily deformed austenite has the same crystallographic characteristic as the typical upper bainite has. Those refinement of bainitic ferrite plates leads to complicate an appearance of the microstructure.
A quantitative analysis reveals the length of bainitic ferrite plates, after 75%-deformation, is consistent with the subgrain size estimated from previous study in low carbon steels. Therefore, subgrain boundaries introduced by heavy deformation are considered to play an important role in refinement of bainite structure, namely they will stifle the growth of bainitic ferrite plates and also supply further nucleation sites of them.